Fouling of processing equipment is inherent to some petrochemical processes. For example, dilution steam systems (DSSs) used in the production of ethylene can be subject to fouling. The dilution steam system of ethylene plants may include a quench water tower (QWT), quench water settler (QWS), process water stripper (PWS), and dilution steam generator (DSG). When this equipment is fouled, it affects the equipment's performance, which in turn affects overall plant efficiency and productivity.
The degree to which processing equipment is fouled relates to the process in which such equipment is used. For instance, the process for producing ethylene may involve steam cracking hydrocarbon feedstocks such as naphtha, ethane, and propane. In the steam cracking (pyrolysis) process, the hydrocarbons are superheated in a furnace to temperatures as high as 750-950° C. For the cracking process, the dilution steam generator supplies dilution steam to the reactor to reduce the partial pressure of the hydrocarbons. The superheated hydrocarbons are then rapidly cooled (quenched) to stop the reactions after a certain point (e.g., to prevent further cracking to methane). The quenching of the superheated gas in many processes is carried out using water in the quench water tower. The superheated cracked gas (including ethylene) is flowed into the bottom of the quench water tower and, at the same time, water is sprayed into the top of the quench water tower. As the water in the quench water tower falls, it makes contact with the upwardly flowing superheated cracked gas and, in that way, cools the superheated cracked gas (that includes ethylene) and dilution steam.
Because of the direct contact between the superheated cracked gas in the quench water tower and the condensation of the dilution steam, the water flowing from the quench water tower is mixed with condensed hydrocarbons (referred to as pyrolysis gasoline). Pyrolysis gasoline may include components such as aromatics, olefins, and diolefins, among others. In the quench water tower, the pyrolysis gasoline and water mixes to form an emulsion. Thus, the quench water tower effluent stream flowing from the bottom of the quench water tower includes an emulsion having a water phase and a hydrocarbon phase. The emulsion is particularly difficult to break. In other words, the emulsion is stable because, once the emulsion is formed, the water does not easily separate from the pyrolysis gasoline.
To facilitate the separation of the water from the pyrolysis gasoline, the quench water tower effluent stream is flowed from the quench water tower to the quench water settler. At the quench water settler, the quench water effluent stream (including the emulsion) is settled and water is drawn off from the bottom of the quench water settler. Then, the water from the quench water settler is sent to the process water stripper. The process water stripper strips the water of acid gases and dissolved hydrocarbons. After being stripped in the process water stripper, the water is routed to the dilution steam generator. The water that circulates from the dilution steam generator to the furnace, to the quench water tower, to the quench water settler, to the process water stripper, and back to the dilution steam generator is referred to as process water, which circulates in a quench water tower loop.
Because the emulsions in the quench water tower tend to be stable, the process water may carry a large amount of hydrocarbons to the process water stripper. These hydrocarbons can cause fouling of the process water stripper. The dilution steam generator may also foul because of hydrocarbon carry-over. Further, process water that flows from the bottom of the quench water tower and the quench water settler can contain traces of styrene as well as oligomers of styrene that form in the water as a result of the long residence time of the water recycle in the quench water tower loop. These oligomers grow further at process water stripper conditions and generally cause fouling in the dilution steam system.
Fouling materials usually have low thermal conductivity and, thus, are a major resistance to heat transfer in processing equipment. Consequently, fouling is of particular concern in heat exchange equipment. When heat exchangers foul, their heat exchanging capacity decreases.
Fouling at the bottom of the process water stripper and in the dilution steam generator preheaters can lead to poor energy efficiency and, in a worst case scenario, to a plant shutdown if excessive fouling sufficiently restricts flow of process water in the quench water tower loop. Fouling of the dilution steam generators can cause cycles of concentration of the dilution steam generator to be low (e.g., 4-5 cycles), which can cause water, energy, and/or chemical losses.
A common method of solving the fouling problem involves the use of emulsion breakers to improve pyrolysis gasoline/water separation in the quench water tower, or quench water settler, or both. Another method for solving the fouling problem is to inhibit polymerization within the quench water tower loop using stable free radical (SFR) type of inhibitors or anti-oxidant. This helps to inhibit the formation of oligomers and thus improves the quality of the water entering the dilution steam system. A further method for solving the fouling by hydrocarbons is to apply a dispersant in the process water stripper. However, this method has limited effect when the amount of hydrocarbons in the water is high. While these methods of preventing fouling are effective to varying degrees, they can be time consuming and/or costly to implement. Further, their efficiencies at removing or preventing fouling tend to be low.